1. Field of the Invention
The present invention relates to a spool valve and pilot interface assembly whereby a spool element in the spool valve is manipulatable to selectively wipe off of sealing engagement a sphere held onto its seat by differential pressure, the movement of the spool and the wiping action causing substantially immaterial resistance to spool movement at the ball-seat interface. The pilot for the spool valve receives a fluid detection signal, which, upon decrease, shuttles the spool to reverse flow through the spool valve. Preferably, the spool valve receives hydraulic control fluid, while the pilot assembly is responsive to pneumatic signal.
2. Description of the Prior Art
As a safety precaution, oil and gas wells are required to contain one or more downhole safety valves, which are typically of ball-type construction and are manipulated between open and closed position by variation of hydraulic control pressure. The safety valve is manipulatable to closed position when detection devices immediate the well respond to an unfavorable condition, such as an increase in temperature, indicating a fire, or by abnormal increase or decrease in well production flow, indicating a flow line plug or rupture. The monitoring devices transmit the abnormal signal to a device which causes automatic loss of hydraulic control fluid to the safety valve, and the safety valve is manipulatable to closed position.
Interfacing mechanisms heretofore utilized have incorporated pressure unbalanced poppet-type valving mechanisms because spool valves having very close tolerance metal sleeves result in high friction being encountered across the valve as the head is shifted with respect to its seat. Thus, it would be desirable to have a bi-stable valving mechanism which is not pressure unbalanced and which does not encounter friction as the head is moved relative to the seat.
Some prior art hydraulic-pneumatic interface valves, that is, an assembly operating one of pneumatic and hydraulic control source which is responsive to the other of pneumatic and hydraulic source in a second signal, are pressure unbalanced and require pneumatic signal to allow hydraulic pressure to the downhole safety valve. In these systems, the pneumatic source is typically tapped from the well, so that when the well is shut in, there is no source of control fluid to reopen the valves. This, in turn, has required utilization of mechanical defeat mechanisms or hand pumps to activate the system to manipulate the valve again to open position. These start-up techniques, if not mechanically released, will defeat the entire safety system.
Valving mechanism heretofore known and utilized have required the incorporation of numerous valves and complicated circuitry. The multiplicity of valves has been utilized to interface many sensing signals, usually at a minimum of one valve per signal. In these systems, manual valves are required to by-pass sensing signals for start-up. Additionally, other manual valves are required to shut down the system. Frequently, hand pumps or mechanical defeat mechanisms have been required to start up the systems. Not only are these valving systems complicated, but they are expensive, result in frequent service problems, and, because of their complexity, have a higher probable failure rate, and are thus unreliable.
Some prior art valving means have utilized spherical elements which essentially act as a valve head which is selectively movable onto and off of an adjacent seat by means of a spool element which is shuttled to manipulate a spherical element. For example, U.S. Pat. No. 2,574,335 discloses two sets of two ball elements each which are opened and closed at the same time in a fluid pressure motor device for control systems. The shuttle device contains a conical carrier surface manipulating a first ball on the conical or bevelled surface to move an enlarged ball thereon between open and closed position within flow passageways.
U.S. Pat. No. 2,891,518 utilizes spherical or ball elements which are not normally free traveling or sealingly engageable upon a seat by differential pressure caused by fluid flow.
U.S. Pat. No. 2,967,544 also is typical of the prior art utilizing spherical elements as a valve head. This patent utilizes compressed spring elements and a mechanical manipulator to shift the balls between opened and closed position with respect to their ports. The spherical or ball elements are not carried by a spool which is shuttleable.
U.S. Pat. No. 3,007,492 discloses a pilot valve for fluid flow systems utilizing a spherical or ball element as the valve head. The ball elements are contained within a shuttleable spool element which is only shuttleable by exertion of a spring element in one direction. A piston head in a piston chamber is utilized to shuttle the spool in the opposite direction against the spring.
U.S. Pat. No. 3,053,279 discloses a directional valve containing a plurality of ball elements manipulatable by a spool to control flow from a second passageway to a first passageway. In one position of the spool, all of the balls are positionable on each of the respective seats. This valve is utilized to control flow from a second passageway to a first passageway, and flow therebetween is controlled by the positioning of a ball within the first passageway.
The present invention obviates the problem set forth above and differs substantially from the prior art. The present invention allows as many sensing signals as desired to control downhole safety valves, or the like, without affecting manual control. Additionally, the present invention does not require bypass valves, mechanical defeat devices, or hand pumps for system start-up. Additionally, the present invention does not rely upon pressure unbalanced poppet-type valve mechanisms, but provides a bi-stable valve which tolerates exposure to high pressures. Additionally, the present invention provides a pressure balanced system which permits easy manual or mechanical shifting of the valve elements. Moreover, the present invention differs substantially from the prior art by utilization of a spool element containing spherical or ball means carried thereby which are selectively engageable onto and wipable off of their respective seats. In addition, the present invention provides a pilot assembly which may be activated without affecting the positioning of the spool, but which is responsive to shift the spool to reverse flow within the spool element. In the present invention, flow must be controlled by positioning of a plurality of ball elements on a plurality of seats, and the use of one ball element on one seat is insufficient. Moreover, in the present invention, many pneumatic or hydraulic signals may be applied to one hydraulic control device, thus simplifying the entire control system.